Increase in Dynamical Collectivity and Directionality of Orange Carotenoid Protein in the Photo-Protective State

Abstract

Photo-protection is crucial for photosynthesis efficiency. Cyanobacteria have evolved a unique photo-protection mechanism mediated by Orange Carotenoid Protein (OCP). OCP binds a single ketocarotenoid as the chromophore, essential to its photo-protective function. Under strong green-blue (or white) illumination or high chaotrope concentration, OCP converts from the orange state OCPO to the activated or photo-protective red state OCPR. The OCPR facilitates dissipation of excess energy via direct interaction with allophycocyanin (APC) cores of the light-harvesting antenna Phycobilisome (PB). Picosecond intramolecular dynamics are critical to the photo-protective conformational switching, energy transfer between the APC and OCP, and energy dissipation. In particular intramolecular vibrations at THz frequencies can both provide efficient access to intermediate state conformations and couple to embedded chromophore vibrations for energy dissipation. Here we characterize global picosecond flexibility using temperature dependent terahertz spectroscopy on OCP solutions. The THz absorbance decreases and structural resilience increases in the photoactive state. The dynamical turn on temperature for picosecond dynamics shifts from 200K in OCPO to 250K in OCPR, signifying a substantial increase in vibrational collectivity and structural stability. To characterize the nature of the intramolecular vibrations in more detail, we employ our recently developed technique Polarization-Varying Anisotropic Terahertz Microscopy (PV-ATM). The technique isolates specific vibrational bands associated with long range collective motions of the protein structure. For the first time we demonstrate intramolecular vibrational changes with photoexcitation. In particular we find an increase in vibrational directionality in the photo-activated OCP in the 60-72 cm−1 and 85-100 cm−1 bands. In addition, the orientation of the vibrational motions switches for the 38-48 cm−1 band. We suggest that the increased dynamical collectivity and directionality changes with photo-state contribute to OCP efficiently binding and interacting with the APC complex to optimize photo-protective function.